1. Field of the Invention
This invention relates to a novel and improved apparatus for processing binary data employing a modified zero modulation data code.
2. Description of the Prior Art
Run length limited (RLL) codes employing variable length words are known in the art, such as is disclosed in U.S. Pat. No. 3,689,899 to P. A. Franaszek. The RLL code is one in which succeeding binary 1's of each encoded bit sequence are separated by no fewer than a (d) number, and no more than a (k) number of binary 0's, the choice of (d) and (k) depending upon the specific code that is being used. The coded data is processed in groups or words of variable length, and the lengths of the encoded words have a constant ratio to the respective lengths of their corresponding bit strings, thus ensuring a constant data transmission or data recording rate.
One type of RLL code is a zero modulation (ZM) code that is disclosed in U.S. Pat. No. 3,810,111 filed in behalf of A. Patel, entitled "Data Coding With Stable Base Line for Recording and Transmitting Binary Data." The ZM code is useful in a circuit that employs an element which is sensitive to a DC signal component, and when there is an imbalance of polarity. Elements of this type are capacitors and inductors that are used as integrators or transformers, and which experience a charging, or increase of voltage or current. The "charge" has a tendency to degenerate or level off, resulting in nonlinear variations in the clock and data signal waveforms, with resultant degradation of the data signal and loss of synchronism.
The ZM code overcomes this problem. In the ZM code, each data digit is encoded as a pair of binary digits, and the binary digit pair is converted to an NRZI waveform. The ZM encoding as described in the Patel patent allows no more than three successive 0 digits between succeeding 1 digits, and a clock transition is produced in at least one of every two adjacent digit periods.
Also backward and lookahead parity functions are used to determine whether a particular sequence of data digits is to be encoded. For such purpose, a network requiring an (n) number of bits lookahead is used, with parity bits inserted for every number of (n) bits.
It would be desirable to have a system that employs a ZM code with all its attendant advantages, but with lesser constraints and with less complex and less expensive hardware.